The present invention relates to a control valve for variable displacement compressors to control displacement.
A refrigeration circuit of a typical vehicle air-conditioning system includes a condenser, an expansion valve, which functions as a depressurizing device, an evaporator and a compressor. The compressor draws refrigerant gas from the evaporator and compresses the gas. The compressor then discharges the gas to the condenser. The evaporator performs heat exchange between the refrigerant in the circuit and air in the passenger compartment. Heat from air that flows about the evaporator is transferred to the refrigerant flowing through the evaporator in accordance with the thermal load or the cooling load. The pressure of the refrigerant gas at the outlet of the evaporator represents the magnitude of the thermal load.
A vehicle variable displacement swash plate type compressor has a displacement control mechanism for setting the pressure (suction pressure Ps) in the vicinity of the outlet of the evaporator to a predetermined target suction pressure. The mechanism adjusts the compressor displacement by changing the inclination angle of the swash plate such that the flow rate of refrigerant corresponds to the cooling load. To control the displacement, a control valve is used. The control valve includes a pressure sensing member, which is a bellows or a diaphragm. The pressure sensing member detects the suction pressure Ps. A valve opening is adjusted in accordance with the displacement of the pressure sensing member, which changes the pressure in a crank chamber, or crank pressure Pc.
A simple control valve that imposes a single target suction pressure cannot control the air conditioning performance accurately. Therefore, an electromagnetic control valve that changes a target suction pressure in accordance with an external current has been introduced. Such a control valve includes an electromagnetic actuator such as a solenoid. The actuator changes a force acting on a pressure sensing member in accordance with an external current to adjust the target suction pressure.
A typical vehicle compressor is driven by an engine. The compressor consumes a significant amount of the power (or the torque) of the engine. Therefore, when the load on the engine is great, for example, when the vehicle is accelerating or moving uphill, the compressor displacement is minimized to reduce the engine load. Specifically, the value of current supplied to the electromagnetic control valve is controlled for setting the target suction pressure to a relatively great value. Accordingly, to increase the actual suction pressure toward the target suction pressure, the control valve operates such that the compressor displacement is minimized.
A graph of FIG. 14 illustrates the relationship between a suction pressure Ps and the displacement Vc of a compressor. The relationship is represented by multiple lines in accordance with the thermal load in an evaporator. Thus, if a level Ps1 is set as a target suction pressure Pset, the actual displacement Vc varies in a certain range (xcex94Vc in FIG. 14) due to the thermal load. For example, when an excessive thermal load is applied to the evaporator, an increase of the target suction pressure Pset may not decrease the engine load. That is, even if the target suction pressure Pset is raised, the compressor displacement Vc will not be lowered to a level that reduces the engine load unless the thermal load on the evaporator is relatively small.
The suction pressure Ps represents the thermal load on an evaporator. The method for controlling the displacement of a variable displacement compressor based on the suction pressure Ps is appropriate for maintaining the temperature in a vehicle compartment at a comfortable level. However, to quickly decrease the displacement, displacement control that is based only on the suction pressure Ps is not always appropriate. For example, displacement control based on the suction pressure Ps is not suitable for the above described displacement limiting control procedure, in which the displacement must be quickly decreased to make the engine power available for acceleration.
Accordingly, it is an objective of the present invention to provide a control valve that quickly and reliably changes the displacement of a compressor regardless of the state of the thermal load on an evaporator.
To achieve the above objective, the present invention provides a control valve used for a variable displacement compressor in a refrigerant circuit. The compressor includes a crank chamber, a discharge pressure zone, a suction pressure zone, a supply passage for connecting the discharge pressure zone to the crank chamber, and a bleed passage for connecting the suction pressure zone to the crank chamber. The control valve comprises a valve housing, a valve chamber defined in the valve housing. A movable valve body is located in the valve chamber to adjust opening size of the supply passage or the bleed passage. A pressure sensing chamber is defined in the valve housing. A dividing member is located in the sensing chamber to divide the pressure sensing chamber into a first pressure chamber and a second pressure chamber. The pressure at a first pressure monitoring point located in the refrigerant circuit is applied to the first pressure chamber. The pressure at a second pressure monitoring point located in the refrigerant circuit is applied to the second pressure chamber. The dividing member moves in accordance with the pressure difference between the first pressure chamber and the second pressure chamber. A rod has a proximal end and an distal end. The distal end is coupled to the dividing member and transmits the movement of the dividing member to the valve body. The pressure of the crank chamber is changed in accordance with the movement of the dividing member and the valve body to control the displacement of the compressor. The pressure in the vicinity of the distal end of the rod is exposed to the presence of the first pressure chamber or the second pressure chamber. An urging mechanism urges the rod axially with a force that represents a target pressure difference between the two pressure monitoring points.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.